The present invention relates to the fabrication of low cost, in situ, metallic foam components having oriented microstructures and improved mechanical properties such as energy absorption and specific stiffness.
Cellular metals have been available for decades, but new opportunities for cellular metals are emerging for two reasons. First, novel manufacturing approaches have beneficially affected performance and cost, and second, higher levels of basic understanding about mechanical, thermal and acoustic properties have been developed. These provide an integrated pathway between manufacturing and design.
Cellular metals have high stiffness and yield strength at low density relative to other competing materials and systems. That is, the cellular materials may be laminated between opposed sheets of another material. This creates an opportunity for ultra-light structures, with integrally bonded dense face sheets. In addition, cellular metals have large compressive strains achievable at nominally constant stress. This feature provides for high-energy absorption capacity which is advantageous in crash and blast amelioration systems. These materials may be used effectively for either cooling or heat exchange structures. Further, cellular metals incorporated within a design to form sandwich skins can achieve mechanical performance and affordability goals at lower weight than competing concepts.
One method of making metallic foams involves gas expansion in foam casting. Another method for making metallic foams is based on gas expansion in foam casting or powder metallurgy. According to this method, metal powder is mixed with a foaming agent, for example a gas. Gas pressure is derived by either a dispersed particulate such as H2 from TiH2, high pressure generated within an entrapped inert gas, or a gas injected into a liquid metal. This mixture can then be extruded or cast into the structural shape required. It is very difficult to control pore size or orientation using these known techniques.
The powder metallurgical Fraunhofer-process is another method used to create metallic foams. In this method, a foaming agent is added to a metal powder that is then mixed. This mixture can then be compacted or extruded into sheets or bars that can then be formed into the component shape using conventional molding techniques. Again, this process has little control of the pore size or orientation, and it is expensive if used to create geometrically complex parts due to the molds required.
Recently, another method, termed the GASAR process, has been developed that provides a means for control of pore shape and orientation. However, because the process involves the use of molten metals and the injection of gases, it is a technically complex and expensive process. Furthermore, the GASAR process allows the use of only one pore or cell orientation in a component and the shapes of the components are generally limited to plates, rods, and tubes.
While others have developed processes for the fabrication of metallic foam structures with oriented porosity, none of those processes are capable of creating a combination of open and closed cell porosity, nor are they capable of creating components directly from CAD designs. Additionally, control of pore size and pore orientation is difficult. Moreover, conventional processes do not provide for the fabrication of integral structures with a metallic foam skin in a cost-effective manner. Consequently, there is a need for a fabrication process that can produce complex metallic foam components with optimized dynamic mechanical properties in a cost-effective manner.
Further, there is a need to fabricate metallic foam structures that have both open and closed cell porosity. Open porosity is characterized by the amount of surface area that is accessible by a gas or liquid if the structure were to be immersed in it, while closed porosity is the porosity in the structure not accessible to a gas or liquid. The size, distribution, and aspect ratio of close-celled porosity in a foamed material can have a direct effect on its energy absorbing and blast amelioration capability and other mechanical properties such as compressive strength.
The present invention overcomes the problems encountered with conventional methods and compositions by providing an efficient, cost-effective process for preparing complex metallic foam components with optimized dynamic mechanical properties. More specifically, the invention provides methods and compositions for metallic foam components. In particular, the present invention relates to the freeform fabrication of metallic foams to form parts having complex geometries that demonstrate superior mechanical properties and energy absorbing capacity. Metallic foam components can be used in a wide range of applications, for example, applications such as antenna masts and fins, wings, exhaust ducts, electronic chassis components, and ribbed heat diffusers for state of the art lighting fixtures.
Further, this method presents a cost-effective method of producing ultra-lightweight structures directly from CAD designs and offers the ability to manufacture complicated shape prototypes with minimal post-processing steps. In addition, the present invention provides methods and compositions of metallic foams that have both open and closed cell porosity, which has a direct effect on its energy absorbing and blast amelioration capability and other mechanical properties such as compressive strength.
Accordingly, an object of the present invention is to fabricate metallic foam components with optimized mechanical properties in an efficient, cost-effective manner.
Another object of the present invention is to fabricate metallic foam components that display increased energy absorbing and other mechanical properties based on the foam having both open and closed porosity.